Grey herons nest in conspicuous colonies of large stick nests in tall trees, and the same sites are used for decades. That visibility makes them countable in a way most birds are not, which is why a national census of heronries began in Britain in 1928 and has continued nearly every year since.
The resulting series is unusually informative. It shows sharp declines after hard winters, when shallow water freezes and herons cannot reach fish, followed by recovery over several mild years. Cold winter weather is among the factors cited when the index turns down. Because the record predates the modern pesticide era and most other monitoring schemes, it also serves as a baseline against which other pressures can be assessed.
The caution is that nest counts are not population counts. Non-breeding birds are invisible to the survey, and colony boundaries can shift, so the series tracks breeding effort well and total abundance only approximately.